It is often necessary or desirable to measure various parameters of blood, such as temperature, pressure and compositional parameters of blood, i.e., the presence and/or concentration of blood constituents, such as blood gases, hydrogen ions (pH), other electrolytes, glucose, red blood cells and the like. Measurement of compositional parameters of blood can be accomplished in real time using fluorescent sensors. For example, this can be accomplished in an extracorporeal blood loop as shown in Cooper U.S. Pat. No. 4,640,820 and in vivo as disclosed in Lubbers et al Reissue Patent No. 31,879. The system disclosed in this Cooper patent involves a fluid (blood) passageway partly defined by membranes which are permeable to one or more blood constituents. For in vivo sensing, a probe or catheter carrying an appropriate sensor is inserted into a blood vessel of the patient. Because blood vessels are quite small, sensors designed to be inserted in such vessels must be very small. This size constraint may have a detrimental effect on the accuracy of the determination made by the sensor.
Kater U.S. Pat. No. 4,535,786 discloses a method and apparatus for measuring parameters of body fluids, e.g., blood, using sensing electrodes which are calibrated periodically. Kater discloses a system in which the sensing electrodes are located at a relatively large area of the blood path. That is, the cross-sectional area of the blood path is increased relative to the cross-sectional area of this path immediately upstream of the sensing electrodes. Thus, a relatively large volume of blood must be drawn from the patient in order to obtain the desired measurements with the system.
Parker U.S. Pat. No. 4,573,968 discloses a system in which blood drawn from a patient into a cassette comes into direct contact with electrochemical sensors which are mounted along a flow path inside the cassette. The wires leading from the cassette to the analyzer appear to be an integral part of the cassette. Thus, both the cassette and the wires of the Parker system require disposal after use, in order to avoid blood contamination between patients. The Kater system has similar disposal concerns. It would be advantageous to have to dispose of as little as possible in order to minimize costs.
Other patents which have been considered in preparing the application include Kulick U.S. Pat. No. 2,629,399; Cunningham et al U.S. Pat. No. 4,398,542; Koning et al U.S. Pat. No. 4,478,222; and Heitzmann U.S. Pat. No. 4,557,900.